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The present invention relates to a process for the preparation of hydrocarbons from synthesis gas. More particularly, this invention relates to a gas-agitated multiphase reactor system with multiple reaction zones comprising gas-liquid or gas-liquid-solid mixtures that can maximize the production rate while allowing better control of the temperature distribution and better control of the liquid and solid phases in the reactors. Still more particularly, this invention relates to a method for operating a pair of linked gas-agitated slurry bed reactors such that the hydrodynamic behavior and reactor performance of such reactor system are improved compared to that of a conventional slurry bed reactor.
Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of an amount of gas is so much greater than the volume of the same number of gas molecules in a liquefied state, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline, jet fuel, kerosene, and diesel fuel have been decreasing and supplies are not expected to meet demand in the coming years. Fuels that are liquid under standard atmospheric conditions have the advantage that in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen, or steam, or a combination of both to form synthesis gas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch (FT) synthesis, carbon monoxide is reacted with hydrogen to form organic molecules containing carbon and hydrogen. Those molecules containing only carbon and hydrogen are known as hydrocarbons. Those molecules containing oxygen in addition to carbon and hydrogen are known as oxygenates. Hydrocarbons having carbons linked in a straight chain are known as aliphatic hydrocarbons and are particularly desirable as the basis of synthetic diesel fuel.
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification). The molecules react to form hydrocarbons while confined on the surface of the catalyst. The hydrocarbon products then are desorbed from the catalyst and can be collected. H. Schulz (Applied Catalysis A: General 1999, 186, p. 3) gives an overview of trends in Fischer-Tropsch catalysis.
The catalyst may be contacted with synthesis gas in a variety of reaction zones that may include one or more reactors, either placed in series, in parallel or both. Common reactors include packed bed (also termed fixed bed) reactors and slurry bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors comprising catalytic particles sometimes called xe2x80x9cslurry reactorsxe2x80x9d, xe2x80x9cslurry bed reactorsxe2x80x9d or xe2x80x9cslurry bubble column reactors,xe2x80x9d operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Some of the principal advantages of gas-agitated multiphase reactors or slurry bubble column reactors (SBCRs) for the exothermic Fischer-Tropsch synthesis are the very high heat transfer rates, and the ability to remove and add catalyst online. Sie and Krishna (Applied Catalysis A: General 1999, 186, p. 55) give a history of the development of various Fischer Tropsch reactors.
It is clear from the prior art that the performance of a SBCR is a combined result of reaction kinetics, heat and mass transfer, and multiphase hydrodynamics. Jackson, Torczynski, Shollenberger, O""Hern, and Adkins (Proc. Annual Int. Pittsburgh Coal Conf. 1996, 13th (Vol 2), p. 1226) showed experimental evidence of the increase of gas bold up with increase in the inlet superficial velocity in a SBCR for Fischer Tropsch synthesis. Krishna, DeSwart, Ellenberger,. Martina, and Maretto (AIChE J. 1997, 43(2), p. 311) measured experimentally the increase in gas holdup with an increase in the gas velocity and solids concentration in a slurry bubble column in churn turbulent regime. Letzel, Schouten, Krishna and van den Bleek (Chem. Eng. Sci 1999, 54, p. 2237) developed a simple model for gas holdup and mass transfer at high pressure in a slurry bubble column. Numerically, Sanyel, Vasquez, Roy, and Dudukovic (Chem. Eng. Sci. 1999, 54, p. 5071) and Pan, Dudukovic, and Chang (Chem. Eng. Sci. 1999, 54, p. 2481) showed examples of computational fluid dynamic modeling and optimization of a slurry bubble column reactor irrespective of the chemistry. Wu and Gidaspow, (Chem. Eng. Sci 2000, 55, p. 573) show examples of computational fluid dynamics simulations of hydrodynamics of Slurry Bubble Column processes.
Much previous work has been aimed at optimization of the slurry bubble column system for Fischer Tropsch and other chemistries. Stem et al. (Ind. Eng. Chem. Process Des. Dev. 1985 25, p. 1214) developed an axial dispersion model for describing the performance of gas agitated multiphase reactor used for Fiscber-Tropsch synthesis. Saxena (Cat. Rev.xe2x80x94Sci. Eng. 1995, 37, p. 227) gives a review of the detailed experimental findings and theoretical models for the design of a Fischer Tropsch SBCR.
Considerable patent literature examines optimization of Fischer Tropsch Slurry Bubble Column reactors (SBCRs). U.S. Pat. No. 5,252,613 presents a method for improving catalyst particle distribution by introducing a secondary suspending fluid. U.S. Pat. No. 5,348,982 discloses one mode of operation for an SBCR. U.S. Pat. No. 5,382,748 shows the use of a vertical downcomer to promote the uniform catalyst distribution. U.S. Pat. No. 5,961,933 and U.S. Pat. No. 6,060,524 disclose that optimal operation can be obtained by introduction of liquid recirculation. Despite the significant level of research, there remains a need for an optimized Fischer Tropsch reactor and reactor configuration.
It is noted by Deckwer (Chem. Eng. Sci. 1976, 31, p. 39) that the gas dispersion is important in bubble columns of diameters greater than 0.5 m, as it may have a strong influence on conversion. It is found that the gas dispersion is a function of the gas holdup, superficial gas velocity, and reactor diameter. In the gas-liquid-solid three-phase reactor, the gas holdup depends on many factors such as gas and liquid velocities, gas distributor design, column geometry, physical properties of the gas and liquid, particle concentration, and reactor internals. Therefore, the gas dispersion coefficient is also a complicated function of these design and operating parameters. Usually, it is necessary to perform an in situ measurement to determine the dispersion coefficient at a given condition.
It is known that the flow patterns of individual phases can affect the reactor performance. Plug flow and well-mixed flow are two extreme flow patterns for reactor systems. In plug flow, there is no backmixing within the reactor, and the composition of the reactants varies with the position within the reactor. By contrast, in a well-mixed system, the composition of the slurry is similar at every point within the reactor. The dimensionless Peclet number, Pe, can be used to represent the degree of backmixing in plug flow. In general, it can be said that higher Peclet numbers indicate less backmixing, i.e. approaching plug flow, while better-mixed flow regimes are associated with lower Peclet numbers. Hence, the highest Peclet numbers will occur when flow in the reactor approximates plug flow.
A Peclet number can be calculated for each phase in a slurry bubble column reactor. Thus, the type of flow of the gas phase in the reactor can be described by the gas Peclet number, which has the form PeG=UGL/DG, where UG is the superficial gas velocity, L is the expanded slurry bed height, and DG is the gas dispersion coefficient. Superficial gas velocity is defined herein as the total inlet gas volumetric flow rate at reactor inlet temperature and pressure divided by the cross sectional area of the reactor vessel excluding the area occupied by any internals and is sometimes referred to a xe2x80x9cinlet superficial gas velocity.xe2x80x9d The gas dispersion coefficient, DG, is a function of the superficial gas velocity, gas holdup, and the reactor diameter. For large scale industrial bubble columns, the axial dispersion coefficients of gas and liquid phases can be calculated using correlations proposed by Field and Davidson (Trans. IChemE 1980, 58, p. 228). The change of the gas Peclet number with the superficial gas velocity at three reactor aspect ratios is shown in FIG. 1. As shown in FIG. 1, the gas Peclet number decreases with the increase of the superficial gas velocity for a given reactor aspect ratio, defined by the ratio of reactor height over reactor diameter (L/D). Field and Davidson""s article also presents the correlation for the liquid Peclet number in bubble columns with liquid circulation. The liquid Peclet numbers for the conjoined reactor system of this invention are calculated using these correlations, and the results are presented in FIG. 2. FIG. 2 shows the liquid Peclet number changes with the superficial gas velocity at two liquid circulation velocities. The liquid circulation velocity is defined as the liquid linear velocity in a reaction zone. In the figure, solid lines show the results with positive liquid velocity which corresponds to the upward liquid flow while dash lines show the results with negative liquid velocity which corresponds to the downward liquid flow. It has been found that Fischer-Tropsch fluidized bed reactors operating at conditions approaching plug flow regime typically provide higher productivity for a given gas superficial velocity than reactors operating with a higher degree of backmixing. However the FT slurry bed reactors operating at conditions approaching plug flow regime typically are at low superficial gas velocities and tend to suffer from uneven distribution of the gas, liquid and solid phases and difficulty of temperature control. In particular, one characteristic of many conventional slurry Fischer-Tropsch reactors is that the flow through the reactor tends to have core-annular characteristics in that an outer, annular region of the reactor will have a much lower gas content than that in the inner region. This core-annular flow reduces the effective volume of the reactor because the entire reactor is not operating at the most efficient reaction conditions. The existence of a core-annular flow also causes an accumulation of water in annular regions of the reactor. Further details relating to Peclet number can be found in co-pending and commonly owned U.S. patent application Ser. No. 10/023,258 filed Dec. 14, 2001, and entitled xe2x80x9cSlurry Bed Reactor Operated in Well-Mixed Gas Flow Regime,xe2x80x9d which is incorporated herein by reference.
Hence, despite significant research in the field of fluidized bed reactors, a need persists for a reactor system that will provide high productivity while also providing more even distribution of its gas, solid and liquid phases and allowing a high degree of temperature and reaction control.
It is believed that a significant improvement in the operation of fluidized bed reactors for Fischer-Tropsch synthesis is achievable using the concepts disclosed herein. The present invention provides a pair of fluidized bed reaction zones of relatively similar heights that are in fluid communication with each other. More specifically, two distinct reaction zones having separate slurry beds are coupled by a pair of fluid flow passages that allow fluid to circulate between the reactors. One fluid flow passage is positioned in the upper half of the expanded slurry beds. The second fluid flow passage is positioned in the lower half of the slurry beds. Apart from the provided fluid flow passages, of which there may be more than two, the reaction zones are preferably not in fluid communication. Nonetheless, they may be in physical and/or thermal contact with each other. For example, two reaction zones may comprise two complete reactors or. may comprise two zones defined within a single reactor by means of a divider. Likewise, one reaction zone may surround another reaction zone.
Regardless of the configuration of the reaction zones, by feeding gas into one of the reaction zones at a greater rate than into the second reaction zone, the volume fraction of gas in the first reaction zone, known as the gas holdup, can be made greater than the gas holdup in the second reaction zone. The difference in gas holdup values, in turn, causes a difference in slurry density between the two reaction zones. Because the slurry beds are approximately at the same height, the difference in slurry density will result in the hydrostatic pressure at the bottom of the second reaction zone being greater than the hydrostatic pressure at the bottom of the first reaction zone. This difference in hydrostatic pressures will cause fluid at the bottom of the second reaction zone to flow into the bottom of the first reaction zone. Finally, fluid leveling between the two reaction zones will cause fluid at the top of the first reaction zone to flow into the top of the second reaction zone. By controlling the flow rates of gas through the two reaction zones and of fluid between the reaction zones, the hydrodynamic behavior of such reactor system and the reactor performance are improved compared to that of a conventional gas-agitated slurry bed reactor.
In some embodiments, the conjoined reaction zones are operated such that the liquid and solid phases in each reaction zone exhibit less backmixing than gas phase, while the gas in each reaction zone is maintained in a well-mixed flow regime. More specifically, the present gas-agitated multiphase reactor system comprising two conjoined reaction zones can be operated such that the dimensionless gas Peclet number, calculated as described, is less than 0.2 and more preferably less than 0.175. At the same time, the solid and liquid Peclet numbers defined by PeS=UGL/DS and PeL=UGL/DL (where DS and DL are dispersion coefficients for solid and liquid phases, respectively) are preferably large enough to ensure less backmixing for those phases. In any event, the solid and liquid Peclet numbers are preferably greater than 0.2, more preferably greater than 0.4, and still more preferably greater than 0.6. In still further embodiments, the reactor system may be operated such that the single per-pass CO conversion is between about 35% to 75%.
Hence, a preferred embodiment of the present invention provides a method for the synthesis of hydrocarbons using solid catalysts in a three-phase reactor that gives high catalyst productivity and reactor capacity. The invention provides a design and method for operating a gas-agitated multiphase reactor system comprising two reaction zones, in which the difference in gas superficial velocities is between about 2 to about 45 cm/s. In accordance with various preferred embodiments, the present reactor system comprises at least one reactor stage with recycle or multiple reactor stages, with water stripping and catalyst/wax separation units shared between pairs or groups of reaction zones.
The present invention provides a novel gas-agitated reactor apparatus for gas-liquid or gas-liquid-solid multiphase synthesis comprising two reaction zones of relatively similar heights that are in fluid communication with each other achieved by at least one pair of fluid flow passages that allow a circulation of fluid between the reaction zones. The fluid circulation results in a more uniform distribution of the solids concentration (when applicable), and/or a more even temperature distribution along the axial direction. Additionally the fluid circulation between the two vessels introduces a forced convection, and hence increases the heat transfer coefficient and reduces the heat transfer surface area necessary for either heating or cooling.
Thus, the present invention provides a gas-agitated multiphase reactor system that is effective for enhancing reactor productivity and improving temperature control. For a synthesis using a catalyst as a solid phase, and having reactant(s) and product(s) in gas-liquid phases, this conjoined gas-agitated reactor system minimizes the difference in axial catalyst concentration and in particular for a Fischer-Tropsch synthesis, it lowers the maximum local water concentration therefore lowering catalyst deactivation rate.
According to one preferred embodiment a method for producing hydrocarbonaceous products from synthesis gas in a multiphase catalytic system comprises (a) providing a reactor system comprising separate first and second reaction zones, each reaction zone comprising a liquid phase and a solid phase containing a catalyst, the first and second reaction zones being in fluid communication with each other via a lower fluid flow passage and an upper fluid flow passage, the each flow passage having an inlet and an outlet; (b) feeding a first gas comprising H2 and CO into the bottom of the first reaction zone and a second gas comprising H2 and CO into the bottom of second reaction zone such that the superficial gas velocity in the first reaction zone is greater than the superficial gas velocity in the second reaction zone and a pressure differential exists across at least the lower fluid flow passage; (c) creating a liquid circulation from the first reaction zone through the upper fluid flow passage to the second reaction zone and from the second reaction zone through the lower fluid flow passage to the first reaction zone by the pressure differential; and (d) controlling the liquid circulation to achieve a desired liquid circulation velocity.
The resulting slurry circulation velocity is preferably between 0.5 and 20 cm/s. In a particularly preferred embodiment, slurry flows up through the first reaction zone, into the second reaction zone through the upper fluid flow passage, down through the second reaction zone and into the first reaction zone via the lower fluid passage. This fluid circulation patter may be maintained substantially continuously during operation or may be intermittent. The superficial gas velocity in the first reaction zone is preferably between 17 and 60 cm/sec and/or between 2 cm/s and 45 cm/s greater than the superficial gas velocity in the second reaction zone. The superficial gas velocity in the second reaction zone is preferably between 15 and 58 cm/s. The difference in superficial gas velocities between the two reaction zones can be adjusted, such as by adjusting a valve in at least one of the fluid flow passages.
The reactor system preferably contains a Fischer-Tropsch catalyst and water can be removed from the reactor system if desired. Portions of the first and second gases may be converted to hydrocarbonaceous products, which may be removed from the reactor system.
In another embodiment, a gas-agitated reactor system for generating products from a reactant gas comprises a first reaction zone, a second reaction zone, a first fluid flow passage having an inlet and an outlet and providing fluid communication between the first and second reaction zones, a second fluid flow passage having an inlet and an outlet and providing fluid communication between the first and second reaction zones, and at least one gas feed system for feeding a reactant gas into the first and second reaction zones at different superficial gas velocities. The first and second reaction zones preferably contain at least one reaction in common. The first fluid flow passage inlet is preferably placed in the upper half of the first reaction zone and the first fluid flow passage outlet is preferably placed in the upper half of the second reaction zone. Likewise, the second fluid flow passage inlet is preferably placed in the lower half of the first reaction zone and the second fluid flow passage outlet is preferably placed in the lower half of the second reaction zone. The first and second reaction zones are preferably substantially of similar height. The first fluid flow passage is preferably positioned at an angle of between 0 and 70 degrees with respect to a horizontal plane and the second fluid flow passage is preferably positioned at an angle of between 0 and 70 degrees with respect to a horizontal plane.
The reactor system can include an external circulation loop, such as one that comprises a product recovery system, a byproduct removal system, or a regenerator unit. The reactor system can further comprise a flow-controlling valve on at least one of the first and second fluid flow passages. At least one of the first and second fluid flow passages may include a turbulence-inducing internal structure and/or gas collection system in fluid communication with the reaction zones.
According to still another embodiment, a reactor system for producing hydrocarbons from syngas in a three-phase catalytic system in which the catalyst is suspended in a slurry comprises a first reaction zone containing a portion of the slurry, a second reaction zone containing a portion of the slurry, a lower fluid flow passage providing fluid communication between the first and second reaction zones, the lower flow passage having an inlet and an outlet, an upper fluid flow passage providing fluid communication between the first and second reaction zones, upper lower flow, passage having an inlet and an outlet, and at least one gas feed system for feeding syngas into the first and second reaction zones at different inlet superficial space velocities. The rate of flow of gas into the first and second reaction zones is preferably such that the gas Peclet number in each zone is less than about 0.175. Alternatively or in addition, the rate of flow of gas into the first and second reaction zones may be such that the gas Peclet number in each reaction zone is less than about 0.175 and the liquid and solid Peclet numbers in each zone are each greater than about 0.2.
The flow of gas into the first and second reaction zones is such that slurry flows up through the first reaction zone, into the second reaction zone through the upper fluid flow passage, down through the second reaction zone and into the first reaction zone via the lower fluid passage.
In still another embodiment, a reactor system for producing hydrocarbons from syngas in a three-phase catalytic system in which the catalyst is suspended in a slurry, comprises a first reaction zone containing a portion of the slurry, a second reaction zone separated from the first reaction zone and containing a portion of the slurry, an upper fluid flow passage providing fluid communication between the first and second reaction zones, the upper flow passage having an inlet and an outlet, upper and lower fluid flow passages providing fluid communication between the first and second reaction zones, each flow passage having an inlet and an outlet, at least one gas feed system for feeding syngas into the first and second reaction zones at different inlet superficial space velocities, a water-stripping apparatus in fluid communication with at least one of the reaction zones, a liquid product removal system in fluid communication with at least one of the reaction zones, and a gas collection system in fluid communication with the reaction zones. As above, the superficial gas velocity in the first reaction zone may be at least 2 cm/sec greater than the inlet superficial gas velocity in the second reaction zone and/or the rate of flow of gas into the first and second reaction zones may be such that the gas Peclet number in each zone is less than about 0.175.